Semiconductor device having features to prevent reverse engineering

ABSTRACT

A ROM circuit includes a first N channel transistor having an output and having device geometry and device characteristics adapted to bias the output at a predetermined level when a P channel circuit is connected to the first N channel transistor; a pass transistor connected between the output and a data bus, the pass transistor connected to a word line, the word line adapted to turn ON the pass transistor when the word line is asserted; and the P channel circuit connected to the data bus and adapted to provide leakage current to charge a gate in the first N channel transistor when pass transistor is turned ON.

The present application is a continuation of U.S. patent application Ser. No. 13/739,463, filed on Jan. 11, 2013, which is a continuation-in-part of U.S. patent application Ser. No. 13/663,921, filed on Oct. 30, 2012, which is a continuation of U.S. patent application Ser. No. 13/194,452 filed on Jul. 29, 2011, which claims the benefit of U.S. Provisional Application Ser. No. 61/494,172 filed Jun. 7, 2011, each of which is incorporated by reference herein in its entirety.

BACKGROUND

It is desirable to design an electronic chip that is difficult to reverse engineer to protect the circuit design. Known reverse engineering techniques include methods for tearing down layers of the chip to expose the logic devices.

Semiconductor teardown techniques typically involve imaging a device layer, removing the layer, imaging the next layer, removing the layer, and so on until a complete representation of the semiconductor device is realized. Layer imaging is usually accomplished using an optical or electron microscope. Layer removal can be done by using physical means such as lapping or polishing, by chemical means by etching specific compounds, by using a laser or a focused ion beam technique (FIB), or by any other known method capable of removing the layers. FIG. 1 shows some of the semiconductor layers and regions that are imaged by the teardown reverse engineering technique.

Once the semiconductor device teardown is complete and the imaging information is gathered, the logic function of the device can be re-constructed by using diffusion, polysilicon, and well areas to define the MOS devices used to create logic gates, and the metal layers to define how the logic gates are interconnected. FIG. 2 shows how the semiconductor layers define the MOS device.

U.S. Pat. No. 7,711,964 discloses one method of protecting logic configuration data. The configuration data for the logic device is encrypted and a decryption key is encrypted using a silicon key. The encrypted decryption key and configuration are transferred to the logic device. The silicon key is used to decrypt the decryption key which is then used to decrypt the configuration data. One problem with this method is that the chip is not protected against physical reverse engineering as described above.

Many other cryptography techniques are known. But, all cryptographic techniques are vulnerable to the conventional teardown techniques.

Disclosed is a method for designing a semiconductor device that is resistant to these techniques. The semiconductor device includes a physical geometry which is not clearly indicative of the device's function. For example, the semiconductor device is designed where two or more types of logic devices have the same physical geometry. When the teardown method is performed two or more devices will show the same physical geometry, but, these two or more devices have different logic functions. This prevents the person performing the reverse engineering to determine the logic functions by the known methods of observing the geometry of the devices.

Employing the disclosed method and device will force the reverse engineer to employ more difficult techniques. These techniques are more time consuming, more expensive, and more likely to have errors.

SUMMARY

The present method and device presents a semiconductor device that it is difficult to reverse engineer using known techniques.

In one aspect, a ROM circuit includes a first N channel transistor having an output and having device geometry and device characteristics adapted to bias the output at a predetermined level when a P channel circuit is connected to the first N channel transistor; a pass transistor connected between the output and a data bus, the pass transistor connected to a word line, the word line adapted to turn ON the pass transistor when the word line is asserted; and the P channel circuit connected to the data bus and adapted to provide leakage current to charge a gate in the first N channel transistor when pass transistor is turned ON.

One device is an electronic element including a first device and a second device. The first device has a first geometry and a first characteristic and the second device has a second geometry and a second characteristic. The first geometry and the second geometry are the same and the second characteristic is different than the first characteristic. The electronic element may include additional devices. The devices may be active devices or they may be a silicided poly resistor and a non-silicided poly resistor.

A second device is an electronic circuit including a first logic device and a second logic device. At least one of the first logic device and the second logic device is comprised of a first device having a first geometry and a first characteristic, and a second device having a second geometry and a second characteristic. The first geometry and the second geometry are the same and the second characteristic is different than the first characteristic.

A method of manufacturing a semiconductor device that is resistant to reverse engineering is provided. The method includes providing one or more invisible bias generators having a first device having a first geometry and a first characteristic, and a second device having a second geometry and a second characteristic, wherein the first geometry and the second geometry are the same and the second characteristic is different than the first characteristic. Multiple logic devices are provided and one or more invisible bias generators are randomly distributed within the logic devices.

A method of designing a semiconductor device that is resistant to reverse engineering is provided. The method includes providing one or more invisible bias generators having a first device having a first geometry and a first bias voltage, and a second device having a second geometry and a second bias voltage, wherein the first geometry and the second geometry are the same and the second bias voltage is different than the first bias voltage. The method also includes providing multiple logic devices; and randomly distributing within the logic devices the one or more invisible bias generator.

Another method of manufacturing a semiconductor device that is resistant to reverse engineering is provided. The method includes providing a substrate, providing a first metal layer, wherein outputs for electronic devices are located on the first metal layer. The method also includes providing a second metal layer, wherein gates for the electronic devices are located on the second metal layer, wherein the first metal layer is located below the second metal layer and it is necessary to remove the second metal layer in order to test the level of the outputs.

These and other features and objects of the invention will be more fully understood from the following detailed description of the embodiments, which should be read in light of the accompanying drawings.

In this regard, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be used as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention;

FIG. 1 illustrates semiconductor layers and regions that are imaged by the teardown reverse engineering technique;

FIG. 2 illustrates how the semiconductor layers define the MOS device;

FIG. 3 illustrates a circuit that is resistive to conventional reverse engineering techniques;

FIG. 4 illustrates a circuit configuration using a level shifter;

FIG. 5 illustrates a second configuration using a level shifter;

FIG. 6 illustrates a circuit configuration without a comparator;

FIG. 7 illustrates a second circuit configuration without a comparator;

FIG. 8 illustrates an circuit configuration having six active devices;

FIG. 9A illustrates a multiplexer using the disclosed techniques;

FIG. 9B illustrates a second embodiment of a multiplexer using the disclosed techniques;

FIG. 10 illustrates the implementation of a “NAND” logic function;

FIG. 11 illustrates the implementation of a “NOR” logic function;

FIG. 12 illustrates the implementation of a “INVERT” logic function;

FIG. 13 illustrates the implementation of a “BUFFER” logic function;

FIG. 14 illustrates the implementation of a “XOR” logic function;

FIG. 15 illustrates the implementation of a “XNOR” logic function;

FIG. 16A illustrates an IBG device having active components;

FIG. 16B illustrates alternative embodiments of IBG devices having active components;

FIG. 17 illustrates a circuit comprised of resistors;

FIG. 18 illustrates a side view of a silicon wafer having active devices;

FIG. 19 shows 2 transistor (2T) IBG ROM circuit in accordance with one aspect of the present invention;

FIG. 20 shows a 2×2 array of a 2T IBG ROM in accordance with the present invention;

FIG. 21 shows a functional block diagram of a 2T architecture ROM system in accordance with the present invention;

FIG. 22 shows an alternate embodiment of a 2T IBG ROM circuit in accordance with the present invention;

FIG. 23 shows 3 transistor (3T) IBG ROM bit-pair circuit in accordance with one aspect of the present invention; and

FIG. 24 shows a functional block diagram of a 3T architecture ROM system in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Many semiconductor processes that contain logic functions provide different types of metal-oxide-semiconductor (MOS) devices to be used in different environments. For example, one device can operate only at lower voltages and can be sized to minimum geometry. Another device can operate at higher voltages and cannot be sized to minimum geometry. Using this type of device allows the semiconductor device to interface to external signals that are higher in voltage when compared to the internal minimum sized devices.

The type of MOS device in the previous example is typically controlled by the electrical characteristics of the diffusion material. These characteristics are changed by slightly altering the atomic structure of this material by using an ion implant dose and energy. This process is normally described as “doping”. This slight change of electrical properties cannot be detected by the conventional reverse engineering teardown techniques.

In order to provide a device that is resistant to these reverse engineering techniques, an invisible bias generator (IBG) has been developed. An IBG may be defined as an electronic device having at least two internal devices where the physical geometries of the internal devices cannot be used to determine the operating characteristics of the IBG.

One example of an IBG is a device where both internal devices have the same geometry but operate differently. For example, the first device may be a transistor that operates at a first voltage level and the second device is a transistor that operates at a different voltage level. In another example, the first device is a silicide resistor while the second device is a non-silicide resistor. In another example, conductive ink is used to create an electronic circuit and the amount of conductive material in the ink is changed between two of the elements.

Another example of an IBG is a device where both internal devices have different geometries but have the same operating characteristics. For example, the first device may be a transistor that operates with first characteristics and the second device is larger a transistor that operates with the same characteristics. In another example, the first device is a silicide resistor while the second device is a non-silicide resistor. In another example, conductive ink is used to create an electronic circuit and the amount of conductive material in the ink is changed between two of the elements.

Another example of an IBG circuit includes devices having multiple possible geometries and multiple possible operating characteristics, with no apparent correlation existing between a given geometry and an operating characteristic.

FIG. 3 illustrates an exemplary IBG circuit 300 that provides an effective deterrent to semiconductor device teardown techniques. The circuit 300 includes a first IBG device comprising a P-channel device 301 and an N-channel device 303 which are connected in series between a power source (VCC) and a ground. A second IBG device comprises a P-channel device 302 and an N-channel device 304 also connected in series between VCC and ground. In one aspect of the present invention, the devices 301-304 may comprise MOS transistors. In a preferred embodiment, the devices 301-304 may also exhibit identical device geometry. The gates on the P-channel devices 301, 302 are floating as they not provided with an input signal (floating gates) and are charged via leakage current to a voltage level approximately VCC minus the threshold voltages of the devices 301 and 302, each of the threshold voltages is independent. The gates on the N-channel devices 303, 304 are also floating gates and are charged via leakage current to a voltage level of approximately ground plus the threshold voltages of the devices 303 and 304.

Each device 301-304 may include a conduction channel between a source and a drain of the device. The depth of the conduction channel is determined by the doping levels of the diffusion (also known as implantation) areas of the gates of devices 301-304 which in turn determine the voltage level on the P and N channel device junctions, labeled VA and VB in FIG. 3. In one aspect of the present invention, the devices 301-304 are formed with different doping levels (also called impurity levels) between at least some of the devices 301-304 while maintaining identical device geometry, thus resulting in the device junctions VA and VB having different voltage levels. A comparator 310 detects the voltage levels of VA and VB and based on the difference in these voltage bias levels outputs a logical“1” or “0”. VA and VB can be any voltage level as the logic criteria of the comparator 310 is based on the difference of these voltages. In a preferred embodiment, the circuit of FIG. 3 contains identical geometry for the P and N channel devices 301-304, thus causing the doping level difference between the devices 301-304 to control the difference in the voltage levels of the device junctions VA and VB. For example, if devices 301 and 303 are doped to form low voltage MOS transistors (such as 2.5V, for example) and if devices 302 and 304 are doped differently to form high voltage MOS transistors (such as 3.3V, for example), then device junction VA is at a higher voltage than device junction VB, and the output of the comparator will be a logical “1”. As another example, if devices 301 and 304 are doped to form low voltage MOS transistors, and if devices 302 and 303 are doped to form high voltage MOS transistors, then device junction VA is at a lower voltage than device junction VB, and the output of the comparator will be a logical “0”. The logic function of this circuit is invisible to reverse engineering teardown techniques since the operating voltages of the device junctions VA and VB are controlled by the doping levels and these doping levels are not determinable by conventional techniques.

For semiconductor technologies which provide different types of MOS devices, such as the high and low voltage devices described above, an advantage of the IBG circuit is that it can be easily constructed with current methods. Also, an IBG circuit in accordance with one aspect of the present invention can be used to create a number of different of logic cells by varying the number of high voltage devices and low voltage devices.

FIG. 4 shows an exemplary circuit 420 including an IBG and a level shifter circuit which produces a logical “1”, or high, output in accordance with one aspect of the present invention. The IBG portion of the circuit 420 comprises transistors 401, 402, 405, and 406 each having a floating gate input. P-channel transistor 401 is connected in series with N-channel transistor 405 at output node 401A, and P-channel transistor 402 is connected in series with N-channel transistor 406 at output node 402A. Each of the transistors of the IBG portion of the circuit can be a P-type or an N-type device. Also each transistor can be a high voltage device or a low voltage device. In a preferred embodiment, a high voltage device operates at 3.3 V while a low voltage device operates at 2.5 V. In an exemplary embodiment, transistor 402 is a low voltage P-type device, transistor 401 is a high voltage P-type device, transistor 405 is a low voltage N-type device, and transistor 406 is a high voltage N-type device, resulting in the voltage level at output node 402A being higher than the voltage level at the output node 401A. For example, transistors 401 and 405 may produce a voltage level of about 100 mV at the output node 401A and transistors 402 and 406 may produce a voltage level of about 1.5 V at the output node 402A. These output levels fall short of being VCC and ground due to transistors 401, 402, 405, and 406 not being fully turned ON or OFF by the charge on their floating gates which are charged by leakage currents. Transistors 401, 402, 405 and 406 are selected to ensure the voltage levels of the output nodes 401A and 402A are such the one voltage level is higher and the other voltage level is lower than the threshold voltage of transistors 407 and 408, described below.

The voltage levels of the output nodes 401A and 402A of the IBG circuit are insufficient to interface directly with digital logic due to the voltage level of the gates of the transistors 401, 402, 405 and 406. To properly interface with digital logic, the signals from the output nodes 401A and 402A are input to a level shifting circuit comprising transistors 403, 404, 407 and 408. Transistors 403 and 404 may comprise low voltage P-type devices and transistors 407 and 408 may comprise low voltage N-type devices. The output node 401A of the IBG circuit is connected to the gate of N-channel transistor 408 of the level shifting circuit and the output node 402A of the IBG circuit is connected to the gate of the N-channel transistor 407 of the level shifting circuit. In an exemplary embodiment, the N-channel transistors may have a threshold voltage of about 700 mV. Thus, the 100 mV voltage level of node 401A which is input to the gate of transistor 408 will turn transistor 408 “OFF” and the 1.5 V voltage level which is input to the gate of transistor 407 will turn transistor 407 “ON”. Thus, transistor 403 will be turned “OFF” and transistor 404 will be turned “ON”, resulting in the output of the level shifting circuit being a logical “1” or HI.

FIG. 4 also shows also an exemplary circuit 430 including an IBG and level shifting circuit which produces a logical “0”, or low, output in accordance with one aspect of the present invention. The IBG portion of the circuit 420 comprises transistors 409, 410, 413, and 414 each having a floating gate input. P-channel transistor 409 is connected in series with N-channel transistor 413 at output node 409A, and P-channel transistor 410 is connected in series with N-channel transistor 414 at output node 410A. Each of the transistors of the IBG portion of the circuit can be a P-type or an N-type device. Also each transistor can be a high voltage device or a low voltage device. In a preferred embodiment, a high voltage device operates at 3.3 V while a low voltage device operates at 2.5 V. In an exemplary embodiment, transistor 409 is a low voltage P-type device, transistor 410 is a high voltage P-type device, transistor 413 is a high voltage N-type device, and transistor 414 is a low voltage N-type device, resulting in the voltage level at output node 409A being higher than the voltage level at the output node 410A. For example, transistors 410 and 414 may produce a voltage level of about 100 mV at the output node 410A and transistors 409 and 413 may produce a voltage level of about 1.5 V at the output node 409A. Transistors 409, 410, 413 and 414 are selected to ensure the voltage levels of the output nodes 409A and 410A are such the one voltage level is higher and the other voltage level is lower than the threshold voltage of transistors 415 and 416, described below.

The voltage levels of the output nodes 409A and 410A of the IBG circuit are insufficient to interface directly with digital logic due to the voltage level of the gates of the transistors 409, 410, 413 and 414. To properly interface with digital logic, the signals from the output nodes 409A and 410A are input to a level shifting circuit comprising transistors 411, 412, 415 and 416. Transistors 411 and 412 may comprise low voltage P-type devices and transistors 415 and 416 may comprise low voltage N-type devices. The output node 409A of the IBG circuit is connected to the gate of N-channel transistor 416 of the level shifting circuit and the output node 410A of the IBG circuit is connected to the gate of the N-channel transistor 415 of the level shifting circuit. In an exemplary embodiment, the N-channel transistors may have a threshold voltage of about 700 mV. Thus, the 1.5 V voltage level of node 409A which is input to the gate of transistor 416 will turn transistor 416 “ON” and the 100 mV voltage level which is input to the gate of transistor 415 will turn transistor 415 “ON”. Thus, transistor 412 will be turned “OFF” and transistor 411 will be turned “ON”, resulting in the output of the level shifting circuit being a logical “0” or LO.

As described above, the circuit 420 gives the “HI” voltage output while circuit 430 gives the “LO” voltage output. The geometry and size of the IBG transistors 401, 402, 405 and 406 of the circuit 420 may be identical to the geometry and size of the IBG transistors 409, 410, 413 and 414 of the circuit 430. The only discernable difference between the two devices is the level of doping between the high voltage transistors and the low voltage transistors. Because the size and the geometry of IBG transistors of device 420 may be identical to the IBG transistors of device 430, it is not possible to determine the difference between these two devices using the conventional reverse engineering teardown techniques.

FIG. 5 illustrates a second example of IBG circuits and level shifting circuits to output a “HI” or “LO” output. Similar to the embodiment shown in FIG. 4, there are 16 transistor devices (501 through 516). Each of the transistors can be a P-type or an N-type device. Also each device can be a high voltage device or a low voltage device. In a preferred embodiment, a high voltage device operates at 3.3 V while a low voltage device operates at 2.5 V. In an exemplary embodiment, transistors 502, 503, 504, 509, 511, and 512 are low voltage P-type devices. Transistor 501 and 510 are high voltage P-type devices. Transistors 505, 507, 508, 514, 515, and 516 are low voltage N-type devices. Transistors 506 and 513 are high voltage N-type devices. Device 520 gives the “HI” voltage output while device 530 gives the “LO” voltage output. The geometry and size of the IBG transistors 501, 502, 505, and 506 of the device 520 may be identical to the geometry and size of transistors 509, 510, 513 and 514 of device 530. The only discernable difference between the two devices is the level of doping between the high voltage transistors and the low voltage transistors. Because the size and the geometry of the IBG transistors of device 520 is identical to that of the IBG transistors of device 530 it is not possible to determine the difference between these two devices using the conventional reverse engineering teardown techniques.

If a semiconductor chip contains an IBG as described in FIG. 4 or FIG. 5, it is extremely difficult for someone trying to reverse engineer the chip using teardown techniques to determine the function of the IBG devices placed on the chip because the geometry of the internal devices are the same.

FIG. 6 and FIG. 7 illustrate examples of IBGs where the voltage levels of the outputs of the circuits are sufficient to directly interface with the devices on a chip. In FIG. 6, device 601 is a high voltage P-type device, such as 3.3 v, device 602 is a low voltage P-type device, such as 2.5 v, device 603 is a low voltage N-type device and 604 is a high voltage N-type device. By connecting the gate of device 601 to the gate of device 602, these devices share the leakage current, resulting in the high voltage device 601 being fully turned OFF and the low voltage device 602 being fully turned ON. Similarly, by connecting the gate of device 603 to the gate of device 604, these devices share the leakage current, resulting in the low voltage device 603 being fully turned ON and device 604 being fully turned OFF. Output node 601A will be sufficiently close to ground to function as a logical “0” and interface directly with other CMOS devices and output node 602A will be sufficiently close to VCC to function as a logical “1” and interface directly with other CMOS devices.

In FIG. 7, device 701 is a low voltage P-type device, such as 2.5 V, device 702 is a high voltage P-type device, such as 2.5 V, device 704 is a low voltage N-type device and 703 is a high voltage N-type device. By connecting the gate of device 701 to the gate of device 702, these devices share the leakage current, resulting in the low voltage device 701 being fully turned ON and the high voltage device 702 being fully turned OFF. Similarly, by connecting the gate of device 703 to the gate of device 704, these devices share the leakage current, resulting in the high voltage device 703 being fully turned OFF and low voltage device 704 being fully turned ON. Output node 701A will be sufficiently close to VCC to function as a logical “1” and interface directly with other CMOS devices and output node 702A will be sufficiently close to ground to function as a logical “0” and interface directly with other CMOS devices.

The geometry and size of the IBG transistors 601, 602, 603 and 604 may be identical to the geometry and size of the IBG transistors 701, 702, 703 and 704 The geometry and size of IBG transistors 601, 602, 603, and 604 may not be identical to each other. The geometry and size of IBG transistors 701, 702, 703 and 704 may not be identical to each other. Additionally, the voltage levels at the gates of the gate connected transistors are equal. The only discernible difference between the two devices is the level of doping between the high voltage transistors and the low voltage transistors. Because the size and the geometry of IBG transistors of FIG. 6 may be identical to the IBG transistors of device FIG. 7, it is not possible to determine the difference between these two devices using the conventional reverse engineering teardown techniques. The IBG shown in FIG. 6 has the same geometry as the IBG shown in FIG. 7 with the only difference being the doping level of some of the transistors. Therefore, if a chip is designed using the IBG illustrated in FIG. 6 and the IBG illustrated in FIG. 7, it is very difficult to determine a difference in the function of the devices made by each design.

The IBG shown in FIG. 6 can include different configurations. In one example, device 601 is a low voltage P-type device, device 602 is a high voltage P-type device, device 603 is a low voltage N-type device and 604 is a high voltage N-type device. In another example device 601 is a high voltage P-type device, device 602 is a low voltage P-type device, device 603 is a high voltage N-type device and 604 is a high voltage N-type device. In another example device 601 is a high voltage P-type device, device 602 is a low voltage P-type device, device 603 is a low voltage N-type device and 604 is a low voltage N-type device. In another example device 601 is a high voltage P-type device, device 602 is a low voltage P-type device, device 603 is a low voltage N-type device and 604 is a high voltage N-type device. There are a total of sixteen configurations possible for a four device IBG.

FIG. 8 illustrates another embodiment of an IBG circuit. Devices 801, 802, 803 are shown as P-type devices and can be any combination of high voltage or low voltage devices. Devices 804, 805, 806 are shown as N-type devices and can be any combination of high voltage or low voltage devices. However, the six devices shown can be any combination of P-type and N-type devices. The six device IBG has a total of 64 possible configurations. Furthermore, an IBG can be comprised of any number of active devices with 2 to the “n” number of combinations, where n is the number of active devices.

FIG. 9A and FIG. 9B illustrate IBG circuits which include multiplexers. Because IBG circuits may be used to select logic functions, it is convenient to implement these circuits in conjunction with digital multiplexers that effectively steer one of two inputs to its output. These IBG based multiplexers select an input base solely on the IBG function. In FIG. 9A, transistors 901, 902, 905 and 906 comprise an IBG circuit and transistors 903, 904, 907 and 908 comprise a multiplexer. In FIG. 9B, transistors 911, 912, 915 and 916 comprise an IBG circuit and transistors 917, 918, 913 and 914 comprise a multiplexer. In FIG. 9A, devices 901 and 906 are 3.3V devices while devices 902, 903, 904, 905, 907, and 908 are 2.5V devices. Inverter 910 provides the inverse of input A and the inverse of input B. In FIG. 9B, devices 912 and 915 are 3.3V devices while devices 911, 913, 914, 916, 917, and 918 are 2.5V devices. Inverter 920 provides the inverse of input A and the inverse of input B. Based on the outputs of the IBG transistors 901, 902, 905 and 906, the multiplexer shown in FIG. 9A selects the B input while the multiplexer shown in FIG. 9B selects the A input based on the outputs of the IBG transistors 911, 912, 915 and 916. The only discernible difference between the two devices is the level of doping between the high voltage transistors and the low voltage transistors. Because the size and the geometry of transistors of FIG. 9A may be identical to the transistors of FIG. 9B, it is not possible to determine the difference between these two devices using the conventional reverse engineering teardown techniques. The IBG shown in FIG. 9A may have the same geometry as the IBG shown in FIG. 9B with the only difference being the doping level of some of the transistors. Therefore, if a chip is designed using the circuit illustrated in FIG. 9A and the circuit illustrated in FIG. 9B, it is very difficult to determine a difference in the function of the devices made by each design. The only difference between these circuits is the configuration of 3.3V and 2.5V devices.

FIG. 10 represents the implementation of a “NAND” logic function and FIG. 11 illustrates the implementation of a “NOR” logic function. In FIG. 10, NAND gate 1010 and NOR gate 1011 output to an IBG based multiplexer 1012, such as the IBG circuit multiplexer shown in FIG. 9A, which selects the output of the NAND gate 1010. In FIG. 11, NAND gate 1110 and NOR gate 1111 output to an IBG based multiplexer 1112, such as the IBG circuit multiplexer shown in FIG. 9B, which selects the output of the NOR gate 1111. These two implementations appear to identical during reverse engineering because the difference between these configurations is the IBG circuit. Without knowledge of the IBG circuit the logic function of these configurations is indeterminate.

FIG. 12 illustrates an implementation of the logic function “INVERT” comprising an inverter 1201 and an IBG based multiplexer 1202, such as the IBG circuit multiplexer shown in FIG. 9A, implemented to select the inverted input. FIG. 13 illustrates an implementation of the logic function “BUFFER” comprising an inverter 1301 and an IBG based multiplexer 1302, such as the IBG circuit multiplexer shown in FIG. 9B, implemented to select the non-inverted input. FIG. 14 illustrates an implementation of the logic function “XOR” comprising an exclusive-or gate 1401, an inverter 1403 and an IBG based multiplexer 1402, such as the IBG circuit multiplexer shown in FIG. 9A, implemented to select the output of the gate 1401. FIG. 15 illustrates an implementation of the logic function “XNOR” comprising an exclusive-nor gate 1501, an inverter 1503 and an IBG based multiplexer 1502, such as the IBG circuit multiplexer shown in FIG. 9B, implemented to select the output of the inverter 1503. As with the previous examples, reverse engineering a chip that has both the “INVERT” of FIG. 12 and the “BUFFER” of FIG. 13 will be difficult to perform because the “INVERT” and the “BUFFER” will have the same appearance. Reverse engineering a chip that has both the “XOR” of FIG. 14 and the “XNOR” of FIG. 15 is difficult because the “XOR” and “XNOR” have the same appearance. As described above, each pair of implementations is indeterminate without knowledge of the logical operation of the IBG circuit based multiplexers.

One advantage of the high voltage/low voltage method of anti-reverse engineering deterrent is that most processes support this distinction. Many implementations are designed to use low voltages internal voltages because as feature size decreases the internal voltage decreases. But, many devices outside of the chip operate at higher voltages and the chips must be able to interface with these devices. Therefore, devices that use higher voltages are still used and being developed. It is possible to for the difference between the low voltage device and the high voltage device to be achieved using small doping changes between P and N devices.

The IBG devices described above include active devices that use the dopant level to control characteristics of the devices. As an example, it is known in a particular process that a doping concentration difference between the 2.5V and 3.3V devices is about 8×E16 atoms/cm3. Structures that have doping density differences below 1×E17 are candidates for IBG design. Examples of IBGs are in FIG. 16.

There are many other combinations of devices that will work besides the 2.5V and 3.3V devices. For example, a 2.5V can be used with a 5V device. A 1.8V device, a 1.5V device, or a 1.2V can be used with a 3.3V device. A 1.2V device can be used with 1.8V or a 2.5V device. A 1.0V device can be used with a 1.8V device, 2.5V device, or a 3.3V device. A 0.85V device can be used with a 1.8V device, a 2.5V device, or a 3.3V device. This list is exemplary only and any combination of devices that can be made with the same physical geometry can be used.

The previous examples illustrate some of the possible implementations of IBG devices using active devices. Another way to achieve an IBG device is to use inactive devices. The IBG can be made using a silicide poly resistor and a non-silicide poly resistor. The first device is used to set the first bias voltage as an active bias voltage and the second device is used to set the set the second bias voltage as an active bias voltage. The difference between the silicide poly resistor and the non-silicide poly resistor will not be apparent to the conventional reverse engineering techniques because the resistors have the same geometry. FIG. 16A illustrates an example of an IBG device. FIG. 16B illustrates other examples of an IBG device.

Polysilicon has fairly high resistivity, about a few hundred μΩ-cm. Resistive devices from polysilicon suffer from this high resistivity because as the device dimension shrinks the resistance of the polysilicon local interconnection increases. This increased resistance causes an increase in the power consumption and a longer RC time delay. Silicides are added to polysilicon devices because the addition of the silicides reduces the resistance and increases device speed. Any silicide that has a much lower resistivity than polysilicon may be used. Titanium silicide (TiSi₂) and tungsten silicide (WSi₂) are two silicides that are commonly used.

Next, one method of forming a silicide device is described. A self-aligned silicide process is conventionally used to from Titanium Silicide. Initially, chemical solutions are used to clean the wafer surface in order to remove contaminants and particles. Next, the wafer is sputtered in a vacuum chamber using argon to remove the native oxide from the wafer surface. Next, a layer of the wafer surface is sputtered to deposit a layer of titanium on the wafer surface. This results in a wafer having the silicon exposed at the source/drain and on top of the polysilicon gate. Next, a titanium silicide is formed on the polysilicon by using a thermal annealing process. For example, annealing can be performed in a rapid thermal process to form titanium silicide on top of the polysilicon and on the surface of the source/drain. Because titanium does not react with silicon dioxide, silicide is formed only where polysilicon directly contacts with titanium. Next, the unreacted titanium is removed by using a wet etch process that exposes the unreacted titanium to a mixture of hydrogen peroxide (H₂O₂) and sulfuric acid (H₂SO₄). Lastly, the wafer is annealed which increases the grain size of the titanium Silicide. The increased grain size improves the wafer's conductivity and reduces wafer's contact resistance.

Another characteristic that can be controlled in the IBG device is the threshold voltage. The threshold of MOS transistors can be controlled by threshold adjustment implant. An ion implantation process is used to ensure that the power supply voltage of the electronic systems can turn the MOS transistor in the IC chip on and off. The threshold adjustment implantation is a low-energy and low current implantation process. Typically, the threshold adjustment implantation is performed before gate oxide growth. For CMOS IC chips, two threshold adjustment implantation processes are needed, one for p-type and one for n-type.

In an IBG device, the process described above can be used to produce resistors that have the same physical dimensions and have different resistance. Conversely, the process can be used to produce resistors that have different geometries and the same resistance.

FIG. 17 illustrates an example of an IBG device implemented by silicide resistors. A voltage source VCC is connected to a circuit having resistors 1701, 1702, 1703, 1704. The resistance of the resistors can be set by the method described above to have two different resistance levels with all of the resistors having the same physical geometry. For example, resistors 1701 and 1704 may be non-silicide resistors while resistors 1702 and 1703 are silicide resistors. In this example if Va is less than Vb then the output of the device is a logic “1.” If Va is greater than or equal to Vb then the output of the device is a logic “0.”

In another embodiment, the devices can be formed using conductive inks. Conductive inks are used to print circuits on a variety of substrate materials. Conductive inks contain conductive materials such as powdered or flaked silver materials.

Conductive inks can be used to implement IBG circuits because the properties of the inks used to print the circuit can be varied to create devices that have different properties. For example, some devices can be printed using conductive ink having an amount of conductive material. Then, conductive ink that has more (or less) conductive material is used to print another portion of the circuit. The circuit then can have devices that look similar and operate differently or look different and operate the same.

One possible method of reverse engineering IBG circuits is to physically measure the devices in the circuit. This can be done using a probe to measure the actual voltage generated by the circuit. In order to thwart this method of reverse engineering, the IBG cells are placed randomly spaced throughout the design. This makes it more difficult to probe the large number of IBG circuits required to reverse engineer the design.

In an alternative embodiment, the types of IBG circuits used are randomly distributed. For example, every third “AND” gate is implemented using an IBG circuit while every fourth “NAND” gate is implemented using an IBG circuit. As the number of devices implemented by IBG circuits is increased, the difficulty in reverse engineering the chip is increased. Additionally, as the number of types of logic devices implemented by IBG is increased, the difficulty in reverse engineering the chip is increased.

In another embodiment, logic blocks are made having logic devices therein. Within each logic block, the IBGs are randomly distributed within the logic block. As a result, different types of logic devices within each logic block are comprised of IBG devices.

In another embodiment, logic blocks are made having logic devices. The designer determines for the logic blocks a critical point and uses an IBG to implement the critical point. The critical point is a point within the logic the block where it is necessary to know the function or output value in order to determine the function of the logic block. Implementing the critical point within the logic block by an IBG is advantageous because this ensures that IBG has maximum effect in preventing reverse engineering. The inability to determine the value of critical point will necessarily prevent the reverse engineer from determining the proper function for the logic block.

For example, if the logic block is an ADDER, replacing a digit in the output can make it impossible to determine the function of the adder. That is because someone trying to reverse engineer the chip monitoring the function of the logic block would expect specific output for an ADDER. When the replaced digit does not give the expected result, it is not determined that the logic block is functioning as and ADDER.

Another advantage of the disclosed system and method is that chip can be designed using standard tools and techniques. Methods of designing a chip are described in the following paragraphs.

A designer creates an overall design for the chip and for logic blocks within the chip. The design is created in a known hardware design language such as Verilog or VHDL. The design is then synthesized into standard logic which converts the design to the optimized gate level. Synthesis may be performed using standard synthesis tools such as Talus Design, Encounter RTL Designer, and Design Compiler. The synthesis maps the logic blocks into standard logic using a standard cell library provided by the supplier. Next, a place and route tool is used to create a physical implementation of the design. This step involves creating a floorplan, a power grid, placing the standard cells, implementing a clock tree, and routing connectivity between cells and input/output pins. Some examples of place and route tools are Talus Vortex, Encounter Digital Implementation, and IC Compiler. Using this process there are various ways to design a chip using IBG devices. One way is to create and characterize one or more new standard cell libraries and use the one or more new standard cells at the beginning of the process. Another approach is to place the IBG devices during the place and route step, either automatically or manually.

Another method of designing a chip is for the designer to create the design using a schematic entry tool. The designer creates a circuit by hand comprising the base logic gates. The designer can optimize the logic functionality using Karnaugh-maps. A layout entry tool is used to create the physical implementation of the design. The designer draws polygons to represent actual layers that are implemented in silicon. Using this approach the designer places IBG devices at any desired location.

Because the above devices result in a design that is difficult to reverse engineer using the conventional tear down techniques, another method may be implemented to reverse engineer the chip. Another known method of reverse engineering is to probe the device while active in order to establish the operating values of the internal devices. In order to perform these methods, the reverse engineer must remove some layers of the wafer to expose the output contacts of the devices. One way to make this technique more difficult is to randomly place the logic devices as described above. Another technique is to design a chip that is physically resistant to these techniques.

FIG. 18 illustrates the layers of a silicon wafer that is resistant to electronic testing of the chip. The wafer has a base layer 1801 that includes the diffusion layer. The oxide layer 1802 is on top of the diffusion layer 1801. The polysilicon layer 1803 is located on top of the oxide layer with the metal layer 1 1804 located thereon. The signal outputs are formed in metal layer 1 1804. Metal layer 2 1805 is located on top of the metal layer 1 1804. The gate connections are formed in metal layer 2 1805. With this layout it is necessary to remove a portion of metal layer 2 1805 in order to probe the signal outputs that are located in metal layer 1 1804. Removing a portion of metal layer 2 1805 disrupts the gate connections of the devices which in turn deactivates the devices. Thus, a reverse engineer trying to probe the device will destroy the functionally of the device during the reverse engineering process.

In many of the techniques described above, the output voltage level of a device is used to determine the operation of the device. But, any other operating characteristic of the device could be used. For example, the rise time of the device, the current drawn, or the operating temperature can be used in the IBG. Also, more than one physical property of the device can be varied. For example, the geometry and the doping level can be controlled to implement an IBG.

Another advantage of the disclosed system and method is that it can be implemented in any type of electronic device. For example, a read-only memory (ROM) can be implemented with the techniques described above and the contents of the memory are protected by the physical implementation of the IBG circuit. This enables a protected memory device without the need for complicated encryption techniques.

An IBG ROM circuit may be a masked memory technology that is highly resistant to hardware reverse engineering techniques. The IBG ROM circuit may be based on bit pairing of N and P channel devices with doping density differences too small to small to be determined by optical differentiation techniques. The IBG ROM increases the complexity and cost of reading out memory using optical reverse engineering processes, thus producing a secure environment for the data stored in the IBG ROM.

FIG. 19 shows 2 transistor (2T) IBG ROM circuit 1900 in accordance with one aspect of the present invention. The 2T IBG ROM circuit 1900 includes a first N channel transistor 1902 having an output node 1904 connected to the source terminal of the N channel transistor 1902. The N channel transistor 1902 is selected to have a device geometry and device characteristics, including doping characteristics, adapted to bias the output node 1904 at a predetermined voltage level indicating a binary 1 or a predetermined voltage level indicating a binary 0 when the N channel transistor 1902 is connected to a P channel device, described in greater detail below. The doping characteristic differences between a binary 1 and a binary 0 are too small to be detected by optical techniques. The gate terminal of the first N channel transistor 1902 is a floating gate and thus not connected to an input signal. The drain terminal of the first N channel transistor 1902 is connected to ground. The 2T IBG ROM circuit 1900 also includes a second N channel transistor 1906 connected between the output node 1904 and a data bus 1908. A word line 1910 is connected to the gate of the N channel transistor 1906. The N channel transistor 1906 operates as pass transistor and is turned ON by the word line 1910. When the pass transistor 1906 is turned ON by the word line 1910, the pass transistor passes the predetermined voltage level of the output node 1904 to the data bus 1908.

A common P channel circuit 1910 is also connected to the data bus and provides the leakage current to charge the floating gate in the first N channel transistor 1902 when the pass transistor 1906 is turned ON. The common P channel circuit 1910 includes a P channel transistor 1912 and a dummy P and N transistor pair 1914 connected in series. The gates of the P channel transistor 1912 and the dummy P transistor are connected, creating the leakage profile required for proper operation of the first N channel transistor 1902 when the pass transistor 1906 is turned ON. The predetermined voltage level will only be present at the output node 1904 when the pass transistor 1906 is turned ON and thus connecting the common P channel circuit 1910 to the transistor 1902 to provide the leakage current for the operation of the N channel transistor 1902.

FIG. 20 shows a 2×2 array of a 2T IBG ROM 2000 in accordance with the present invention. The 2×2 IBG ROM includes four N channel transistors 2002, 2004, 2006 and 2008 and their associated pass transistors 2012, 2014, 2016 and 2018. The four N channel transistors 2002, 2004, 2006 and 2008 have output nodes 2003, 2005, 2007 and 2009. The N channel transistors 2002, 2004, 2006 and 2008 are selected to have device geometries and device characteristics, including doping characteristics, adapted to bias the output nodes 2003, 2005, 2007, and 2009 at predetermined voltage levels indicating a binary 1 or a predetermined voltage level indicating a binary 0 when the N channel transistors 2002, 2004, 2006 and 2008 is connected to a P channel device, described in greater detail below. The doping characteristic differences between a binary 1 and a binary 0 are too small to be detected by optical techniques. Transistors 2002 and 2004 are both part of a first word, and their pass transistors 2012 and 2014 are turned ON by a first word line 2020. Transistors 2006 and 2008 are both part of a second word, and their pass transistors 2016 and 2018 are turned ON by a second word line 2022. The output of pass transistors 2012 and 2016 are connected to a first data bus 2030 and the output of pass transistor 2014 and 2018 are connected to a second data bus 2032. When the word line 2020 is asserted the pass transistors 2012 and 2014 are turned ON and the pass transistors 2012 and 2014 pass the predetermined voltage levels of the output nodes 2003 and 2005 to the data buses 2030 and 2032. When the word line 2022 is asserted the pass transistors 2016 and 2018 are turned ON and the pass transistors 2016 and 2018 pass the predetermined voltage levels of the output nodes 2007 and 2008 to the data buses 2030 and 2032.

A first common P channel circuit 2040 is connected to the first data bus 2030 and operates as the common P channel for transistors 2002 and 2006, and a second common P channel circuit 2042 is connected to the second data bus 2032 and operates as the common P channel for transistors 2014 and 2018. The predetermined voltage level will only be present at the output nodes 2003 and 2005 when the pass transistors 2012 and 2014 are turned ON and thus connecting the common P channel circuit 2040 to the transistors 2002 and 2004 to provide the leakage current for the operation of the N channel transistors 2002 and 2004. Similarly, the predetermined voltage level will only be present at the output nodes 2007 and 2009 when the pass transistors 2016 and 2018 are turned ON and thus connecting the common P channel circuit 2042 to the transistors 2006 and 2008 to provide the leakage current for the operation of the N channel transistors 2006 and 2006.

FIG. 21 shows a functional block diagram 2100 of a 2T architecture ROM system in accordance with the present invention. An address decode 2102 unit receives the address to be read from an external system and decodes this address to select a word line which corresponds the word of data to be read from the IBG N channel device array 2104. Common P channel devices 2106 are connected to each data line output 2104. A read amplifier 2108 amplifies the word of data output to convert the word of data from voltage levels output the array 2104 to levels that correspond to logical “1” and logical “0” in digital logic circuits. The read amplifier transmits the amplified data on a data bus 2110.

FIG. 22 shows an alternate embodiment of a 2T IBG ROM circuit 2200 in accordance with the present invention. In contrast to the 2T IBG ROM circuit 2000 shown in FIG. 20, the gates of the N channel IBG transistors 2002 and 2004, and the gates of the N channel IBG transistors 2006 and 2008, are connected in a bit-pair fashion. Connecting these N channel gates increases the gate capacitance and leakage current of the transistors 2002, 2004, 2006 and 2008 when compared to the 2T IBG ROM circuit 2000. This allows smaller geometry IBG cells having smaller geometry to operate properly and settle faster.

FIG. 23 shows 3 transistor (3T) IBG ROM bit-pair circuit 2300 in accordance with one aspect of the present invention. The 3T IBG ROM circuit 2300 includes a first transistor pair having a P channel transistor 2302 connected in series with an N channel transistor 2304 through an output node 2306. A second transistor pair has a P channel transistor 2308 connected in series with an N channel transistor 2310 through an output node 2312. The gate of transistor 2302 is connected to the gate of transistor 2308, allowing these devices to share leakage current. Similarly, the gate of transistor 2304 is connected to the gate of transistor 2310, allowing these devices to also share leakage current. The transistors 2302 and 2304 are selected to have a device geometries and device characteristics, including doping characteristics, adapted to bias the output node 2306 at a predetermined voltage level indicating a binary 1 or a predetermined voltage level indicating a binary 0. The doping characteristic differences between a binary 1 and a binary 0 are too small to be detected by optical techniques.

An N channel transistor 2314 is connected between the output node 2306 and a data bus 2316. An N channel transistor 2318 is connected between the output node 2312 and a data bus 2320. A word line 2322 is connect to the gate of the N channel transistor 2314 which operates as pass transistor and is turned ON by the word line 2322. The word line 2322 is also connected to the gate of the N channel transistor 2318 which operates as a pass transistor and is turned ON by the word line 2322. When the word line 2322 is asserted, the pass transistors 2314 and 2318 pass the predetermined voltage levels of the output nodes 2306 and 2312 to the data busses 2316 and 2320.

FIG. 24 shows a functional block diagram 2400 of a 3T architecture ROM system in accordance with the present invention. An address decode 2402 unit receives the address to be read from an external system and decodes this address to select a word line which corresponds the word of data to be read from the IBG P and N channel device array 2404. A read amplifier 2408 amplifies the word of data output to convert the word of data from voltage levels output the array 2104 to levels that correspond to logical “1” and logical “0” in digital logic circuits. The read amplifier transmits the amplified data on a data bus 2410.

In another aspect of the present invention, a security shield may be utilized with an array of IBG ROM circuits. An IBG ROM circuit array may include a top metal trace or run that is routed in a serpentine manner over a surface of the array to provide the ground (GND) connections for devices which comprise the array. For example, the security shield may be placed over the second metal layer 1805 of FIG. 18. Any attempt to reverse engineer the array which cuts the security shield will cause the IBG ROM circuits to fail, complicating any circuit measurements during operation. After being repaired, the cuts will exhibit increased DC resistance and thus limit the number of repairs which can be completed successfully.

The many features and advantages of the invention are apparent from the detailed specification. Thus, the appended claims are intended to cover all such features and advantages of the invention which fall within the true spirits and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Accordingly, all appropriate modifications and equivalents may be included within the scope of the invention.

Although this invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made which clearly fall within the scope of the invention. The invention is intended to be protected broadly within the spirit and scope of the appended claims. 

What is claimed is:
 1. A ROM circuit comprising: a first transistor having an output and having device geometry and device characteristics adapted to bias the output at a predetermined level indicating a binary 1 or a binary 0 when a circuit is connected to the first transistor through a pass transistor; the pass transistor connected between the output and a data bus, the pass transistor connected to a word line, the word line adapted to turn ON the pass transistor when the word line is asserted; and the circuit connected to the data bus and adapted to provide leakage current to charge a gate in the first transistor when pass transistor is turned ON.
 2. The ROM circuit of claim 1 wherein the gate of the first transistor is a floating gate.
 3. The ROM circuit of claim 1 wherein the gate of the first transistor is connected to a gate of a second transistor.
 4. The ROM circuit of claim 3 wherein the second transistor includes a second output and has device geometry and device characteristics adapted to bias the second output at a predetermined level when the circuit is connected to the second transistor.
 5. The ROM circuit of claim 1 further comprising a read amplifier connected to the data bus.
 6. The ROM circuit of claim 1 further comprising a security shield disposed over a surface of the ROM circuit.
 7. The ROM circuit of claim 6 wherein the security shield comprises a metal trace extending over a portion of the surface of the ROM circuit, the metal trace connecting the transistors of the ROM circuit to ground. 